1. Field of the Invention
The present invention relates to an "environmental pollution-free" process for hardening the walls of hydrophobic oil-containing capsules in which the walls comprise a hydrophilic colloid complex coacervate. More particularly, it is concerned with a process for hardening the walls of hydrophobic oil-containing capsules using as a hardener an oxidation product of a polysaccharide harmless to animals and humans in order to improve the physical properties of the coacervate wall, in the production of microcapsules employing a coacervation process using at least one hydrophilic colloid which is gellable.
2. Description of the Prior Art
Formaldehyde, glyoxal and glutaraldehyde are well known as hardeners for gelatin walls. However, the use of these aldehydes (in particular, formaldehyde) can be the cause of serious environmental pollution due to their toxicity, stimulative property, etc. In addition, microcapsule walls prepared by using these aldehydes have the defects that their impermeability is poor, they are weak in strength and they are affected by humidity to a great extent.
When oil-containing capsules prepared according to these processes are used for pressure-sensitive recording papers, the following defects exist.
That is, the microcapsule wall is poor in impermeability as shown by subjecting the microcapsules to a water immersion coloring test where the degree of coloration on a color developer sheet is too high. In the water immersion coloring test a color former containing microcapsule sheet and a color developer sheet are immersed for 5 seconds in water with the coated surfaces of the two sheets facing each other, drying in the air and evaluating the degree of coloration on the color developer sheet. Also, since the strength of the microcapusle wall is weak, the microcapsules are easily ruptured by a slight external pressure, which reduces the workability of the microcapsules in handling for processing and printing. Furthermore, since the microcapsule wall is deteriorated by humidity, when pressure-sensitive recording papers are stored under an atmosphere of high humidity, various properties of the pressure-sensitive recording papers are degraded.